Flame detectors may comprise an optical sensor for detecting electromagnetic radiation, for example, visible, infrared or ultraviolet, which is indicative of the presence of a flame. A flame detector may detect and measure infrared (IR) radiation, for example in the optical spectrum at around 4.3 microns, a wavelength that is characteristic of the spectral emission peak of carbon dioxide. An optical sensor may also detect radiation in an ultraviolet range at about 200–260 nanometers. This is a region where flames have strong radiation, but where ultra-violet energy of the sun is sufficiently filtered by the atmosphere so as not to prohibit the construction of a practical field instrument.
Some flame detectors may use a single sensor, for an optical sensor, which operates at one of the spectral regions characteristic of radiation from flames. Flame detectors may measure the total radiation corresponding to the entire field of view of the sensor and measure radiation emitted by all sources of radiation in the spectral range being sensed within that field of view, including flame and/or non-flame sources which may be present. A flame detector may produce a “flame” alarm, intended to indicate the detection of a flame, when the level of combined radiation sensed reaches a predetermined threshold level, known or thought to be indicative of a flame.
Some flame detectors may produce false alarms which can be caused by an instrument's inability to distinguish between radiation emitted by flames and that emitted by other sources such as incandescent lamps, heaters, arc welding, or other sources of optical radiation. Single-wavelength flame detectors can also create false alarms triggered by other background radiation sources, including various reflections, such as solar or other light reflecting from a surface, such as water, industrial equipment, background structures and vehicles.
Various techniques have been developed which are intended to reduce false positives in flame detectors. Although these techniques may provide some improvement in false positive rates, the rate of false positives may still be higher than desired.